There are numerous uses for applying a fluid to a planar substrate. For example, the substrate may have on it sensors or devices for detecting components within the fluid, and/or be treated to selectively bind or react with components within the fluid. Substrates might include solid-state IC sensor chips, glass slides, genomic and proteomic arrays, and or other reagents chemically attached or dried onto the substrate. One challenge to such applications is reliably and easily attaching some type of fluid chamber or flow cell to the substrate.
One use where the methods and apparatus for applying a fluid to a planar substrate is applicable is in “lab-on-a chip” (or LOC) devices. LOC devices use microliter-scale volumes and millimeter-to-micrometer-scale components to replace bench-top chemical and biochemical instrumentation. Several benefits of such devices over standard laboratory systems include reduced consumption of reagents, reduced volume of waste products, easier controlled process parameters, increased reaction time, and more rapid chemical analysis.
One hallmark of LOC systems is the ability to perform a number of individual tests in parallel on a planar surface. For example, a typical planar DNA oligonucleotide microarray may consist of 50 to 200 micrometer-diameter spots deposited with a robotic spotter onto the substrate in a grid pattern. The array can include up to several thousand (cf. 30,000) unique DNA probe sequences and is, operationally, at least several thousands of experiments running in parallel.
A key component of any assay incorporating a biochemical capture surface such as these is the method by which sample containing the target, along with any other required reagents, are delivered to the capture surface. Most often the reagents are delivered in a static fluidic environment, such as a microtiter well. More recently, a variety of microsystems have been developed to deliver the fluid under dynamic (often laminar) flow over planar substrates. For example, see Becker et al. Polymer microfluidic devices. Talanta 56, 267-287 (2001), Mastrangelo, et al, Microfabricated devices for genetic diagnostics, Proc. IEEE 86, 1769-1787 (1998). Bardell, et al., Microfluidic disposables for cellular and chemical detection—CFD model results and fluidic verification experiments. Proc. SPIE 4265, 1-13 (2001), Hofmann, et al., Three-dimensional microfluidic confinement for efficient sample delivery to biosensor surfaces. Application to immunoassays on planar optical waveguides. Anal. Chem. 74, 5243-5250 (2002), Li, et al., Biology on a chip: microfabrication for studying the behavior of cultured cells, Crit. Rev. Biomed. Eng. 31, 423-488 (2003), and Erickson, et al., Integrated microfluidic devices. Anal. Chim. Acta 507, 11-26 (2004). The challenge becomes how to integrate the fluidics along with the chosen detection technology (i.e. electrical, optical, etc.) with these substrates on this small size scale.
Examples of fluidic devices designed to handle multiple samples or assay protocols include inventions by H. J. Rosenberg, U.S. Pat. No. 3,481,659 include Elkins, U.S. Pat. No. 3,777,283, G. Bolz et al., U.S. Pat. No. 4,338,024, Golias. U.S. Pat. No. 4,505,557. Clatch, U.S. Pat. No. 6,165,739, and Wilding, et al., U.S. Pat. No. 6,551,841.
There are also a number of commercially available slides incorporating multiple fluidic compartments or the means to create individual chambers on the slide (e.g., Fisher Scientific, Grace Bio-Labs). Various custom microliter volume flow cells made of quartz or molded from polydimethylsiloxane (PDMS), as well as a multi-well, flow-through hybridization chamber which incubate three whole chips in parallel for magnetic force discrimination assays have been disclosed. See Malito et al., A Simple Multichannel Fluidic System for Laminar Flow Over Planar Substrates. NRL/MR/6170-06-8953; MR-8953, (2006).
In general, the approaches taken by these devices are guided by the applications addressed. For example, devices may isolate separate volumes on a single microscope slide in order to analyze several samples at once (in static volumes). Other devices contain a single channel for the purpose of analyzing individual particles. In general, however, none of these devices, with the exception of the devices disclosed by Clatch, Wilding, Covington and Malito, are appropriate for conducting assays under controlled flow rates. Although the devices by Clatch and Wilding could be used for monitoring different reactions or assay conditions in parallel, the devices as reported require complex semiconductor microfabrication methods, are designed to share reagents from a single reservoir, or the reagents are distributed by uncontrolled capillary action. Covington's device requires several layers of stencil material to form multichannels, and no clear means to connect their devices to fluidic sources is indicated.
Methods and devices currently in use are encumbered by complicated designs and manufacturing methods making them unsuitable for mass production, to be used as a cheap disposable end-product, or to be compatible with standard off-the-shelf pumping and valving components. See, for example, Jolley, U.S. Pat. No. 4,704,255, Manns, U.S. Pat. No. 5,047,215, Shartle, U.S. Pat. No. 5,627,041, Packard et al., U.S. Pat. No. 5,640,995, and Zanzucchi, et al., U.S. Pat. No. 5,755,942.
Another deficiency of most microfluidic systems is that their complicated construction and usage are not conducive for handling as a simple tool that can be routinely assembled and reused by a laboratory technician with the same ease of, say, a standard micropipettor. Brevig et al., Hydrodynamic guiding for addressing subsets of immobilized cells and molecules in microfluidic systems. BMC Biotechnology 2003, 3:10 (Sep. 19, 2005) discloses a simple docking station that provides a mechanical force for sealing a flat substrate (e.g. glass slide) against a single microfluidic cell without any adhesives or bonding strategies. The flow cell was also designed to actively direct the trajectory and control the width of the sample stream using two additional guiding streams. However, manipulating the individual flow rates of the guiding streams adds a layer of complexity to the external fluidic control requirements. Another deficiency is that the dock is only capable of operating a single fluid cell, and hence a single assay.
The assembly of the different layers of the fluidic device, in particular the cover plate that encloses the channels, have relied on mechanisms such as adhesives, thermal bonding under high compression, chemical bonding, hot gas welding, and ultrasonic welding. Of these, adhesives are the dominant means for assembly.
Covington et al., U.S. Pat. No. 6,848,462 discloses an adhesiveless microfluidic device having several microchannel formats dictated by what they describe as stencil layers which can easily be changed to rapid prototype different channel geometries. However, construction of their device could require compressing several stencil layers between at least two thermoplastic cover layers under high pressure and temperature. Alignment pins are required by other incarnations of their device to properly orient the various layers of material.
Ekström et al., U.S. Pat. No. 5,376,252 and Öhman, U.S. Pat. No. 5,443,890 make use of an elastomer spacing layer or injected sealing material that forms a sealed microchannel between at least two cover plates under moderate pressure. In both disclosures, grooves and/or ridges must first be made into the cover plates to stabilize the material. A deficiency with this design is the channel geometry must be permanently defined in the substrates. If a new channel geometry is required, new substrates must be made.